How fast will a 9hp engine go on a little go kart frame?

It is a mantis rototiller engine. (9hp) It has allot of torque! It is going on a little go kart frame. My son weighs around 100pounds.

It depends entirely on the gear ratio and the tire size.

You can fix it for fast acceleration and low top speed with lots of torque

or

slow acceleration and high top speed with less torque.